XKit Preferences: Remove broken Old Sidebar script

[marcustyphoon]

Old Sidebar, of course, doesn't work, but more to the point it still does run this, which is bad:

var account = document.getElementById("account_button");
account.click();

This removes it (without a notification, to avoid user confusion due to an update to New XKit containing the phrase "sidebar" coming during a period of discussion about a related-but-different sidebar).

[AprilSylph]

Not sure if we care enough to also remove it from the non-React version of the "Get Extensions" tab.

Also, will give @nightpool the honour of the final say between doing this and not.

[nightpool]

we could ask @estufar

[marcustyphoon]

Not sure if we care enough to also remove it from the non-React version of the "Get Extensions" tab.

Ah, yeah, good point. To the extent that we still support non-React anything, this PR may as well not enable an "install extension but it immediately silently vanishes" interaction.

[nightpool]

Lived with this installed for 2 days and it was annoying enough that i've decided we should remove it, although i still like the concept and it would be fun to update it to modern react.... my only concern is that if we get rid of it now it might be annoying to add back but it's been broken long enough i think it makes sense to do the silent removal for now and then people can re-add it when it works again (or we can just add it to xrw or whatever)